Magnetic effect of ocean flow revealed

Tiny magnetic fields created by ocean tides have been mapped globally for the first time, after researchers filtered out the signal from satellite data.

Improving the technique so that the magnetic fields created by all the ocean's motions can be picked out would provide a valuable new way to track global ocean circulation and climate change. It would be especially useful for monitoring deep water currents, as these cannot be seen by the current measurements of water colour or wave height.

This is possible - at least in theory - says Robert Tyler, at the University of Washington in Seattle, who created the new map with colleagues in Germany.

"It opens up a new area of research," comments geophysicist Michael Purucker, at the NASA Goddard Space Flight Center, near Washington DC. "But we're not going to know if tracking is possible until we try."

Positive and negative

The Earth's magnetic field is created primarily by the flow of molten iron in the planet's core. But other factors affect the field too, like the magnetisation of rocks in the crust and the flow of the ocean.

Dissolved salts in ocean waters exist as positive and negative ions, like sodium and chloride. As water flows around the planet, these ions are deflected by the Earth's magnetic field - positive ions are pushed one way, negative ions the other.

This builds up volumes of positively and negatively charged water, sometimes on a scale of thousands of kilometres. As the particles move to reduce this charge separation they create loops of electric current in the water. These, in turn, induce magnetic fields.

These induced fields are tiny compared to the Earth's overall field, which is 6000 times stronger. But it is still the biggest part of the Earth's field left to be modelled and mapped by geophysicists. So Tyler's team looked at magnetic field data from the CHAMP satellite to see if they could pick out the ocean's contribution.

Frequency match

They found a signal that matched their computer model of the effect by filtering out tiny fluctuations that had the same frequency as the tides. The biggest fields were generated, unsurprisingly, in parts of the ocean where the tide is strongest.

"Finding the tides is the easiest problem, that's why we started with it," says Tyler. Filtering out the magnetic fields made by other kinds of ocean circulation will be much harder since there is no regular frequency to identify them.

Tyler also hopes that historical data of the Earth's magnetic field - a record from land-based magnetometers stretching back 100 years - could someday be used to infer changes in ocean circulation in the past. "But that's a much harder problem," he admits.

Journal reference: Science (vol 299, p 239)

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